1. Field of the Invention
The present invention relates to image forming devices for forming developer images on transfer media and to image forming devices having an image forming section and a transfer conveying-section.
2. Description of the Related Art
In image forming devices, such as copiers, printers, and facsimile devices, a sheet stored in a sheet cassette is fed to a transfer nip where a toner image is transferred onto the sheet. Subsequently, the sheet is fed to a fixing unit where the transferred toner image is fixed to the sheet. Then, the sheet having the toner image formed thereon is ejected from the device.
When the transfer process is to be performed, a pair of registration rollers provided upstream from the transfer nip stops the sheet, which is fed slantwise from the sheet cassette, and positions the sheet to the proper orientation. The stopped sheet is outputted from a nip formed by the pair of registration rollers in accordance with the timing of the transfer process.
If a large-size paper having low elasticity is used as the sheet on which a toner image is to be formed, the sheet may tend to undulate during the feeding operation, consequently leading to a transfer defect. In light of this, a configuration as shown in FIG. 14 is used.
FIG. 14 illustrates an example of the configuration of a section between a pair of registration rollers and a transfer nip in an image forming device. As shown in FIG. 14, a transfer nip 1003 is formed between a photosensitive drum 1001 and a transfer roller 1002. A registration nip 1006 is formed between registration rollers 1004 and 1005 at a position upstream of the transfer nip 1003. A conveying path 1009, used for feeding a sheet 1000 from the registration nip 1006 to the transfer nip 1003, is formed by an upper guide 1007 and a lower guide 1008. The lower guide 1008 has an uppermost portion 1008a at an uppermost position and slopes therefrom towards the registration nip 1006 and the transfer nip 1003. The uppermost portion 1008a is positioned higher than the registration nip 1006 and the transfer nip 1003.
With this configuration, the sheet 1000 can be curved during the feeding operation so that even if the sheet is large and thin, which means that the sheet has low elasticity and tends to undulate easily, the undulation thereof can be reduced, thereby allowing for a satisfactory transfer process.
However, when a normal sheet or a thick sheet is fed through the image forming device shown in FIG. 14, the elasticity of the sheet may cause the trailing end thereof to spring upwardly as the trailing end comes off the registration nip 1006. This may cause the trailing end to strike the upper guide 1007, resulting in the occurrence of transfer deviation.
As an attempt for preventing such transfer deviation, an image forming device equipped with a trailing-end-springing prevention member near the exit of the registration nip 1006 has been used.
In an image forming device shown in FIG. 15, the upper guide 1007 is provided with a trailing-end-springing prevention member 1010 at a position near the exit of the registration nip 1006. This trailing-end-springing prevention member 1010 prevents the trailing end of a sheet from springing upwardly so as to minimize the occurrence of transfer deviation.
To further minimize the occurrence of transfer deviation in this configuration, it is necessary to reduce the amount by which the trailing end springs upward. In order to achieve this, it is preferable that the trailing-end-springing prevention member 1010 is located where it comes into contact with a sheet at a vertical position as close to that of the registration nip 1006 as possible.
However, if the trailing-end-springing prevention member 1010 is located where it comes into contact with a sheet at a vertical position close to that of the registration nip 1006, when a sheet having high elasticity, such as a thick sheet, is used, the friction between the sheet and the trailing-end-springing prevention member 1010 may lower the feed rate of the sheet. This is problematic in that a resultant transferred image may be compressed into a region that is shorter than the proper feeding distance (referred to as “image shrinkage” hereinafter).